1. Field of the Invention
The present invention relates to an optical data recording and/or reproducing apparatus with a disc-shaped optical memory medium. More specifically, the present invention relates to a two-stage tracking servo system whereby a light beam tracks the desired information track on the optical memory disk.
2. Description of the Prior Art
An optical data recording and/or reproducing apparatus using a laser beam controlled to irradiate the optical memory disk is known as, for example, a laser memory apparatus or a magneto-optical memory apparatus.
In the optical memory apparatus of this type, information data is precisely stored onto and retrieved from the previously determined track provided on the optical memory disk by means of a laser beam tracking system.
In the optical memory apparatus, there is usually an eccentricity of several tens of microns on the track when the optical memory disk is rotating due to the disk eccentricity itself and, in another case, due to a spinning shaft of a disk drive motor.
Under the above mentioned condition, an objective lens is usually shifted from the mechanical center of the lens-carrying pickup when a simple lens tracking system is used for the apparatus. This also offsets an electric tracking signal as optical offset and the optical offset makes the beam track at the off-track position depending on the signal offset level.
To avoid the off-track condition, there have been proposed several systems, among which one example is the eccentricity compensation system in which pickup motion is controlled to compensate for the eccentricity by means of a special correction apparatus whereby the eccentricity is previously measured and stored. Another example is the two-stage tracking servo system in which both objective lens and the carriage wherein the lens is installed are controlled to track the desired track on the optical memory disk in two-stage way depending on the tracking signal.
In the former system, there is required some special correction apparatus for compensating for the eccentricity.
In the latter system, inappropriate timing control for lens tracking and carriage tracking makes it difficult to move the beam from off-track condition to an on-track condition swiftly because the objective lens and the carriage, at the same time undergo, lens tracking respectively and carriage tracking, in the two-stage way.
In other words, not only the objective lens cannot track the nearest track of the beam irradiating position the moment the tracking system tracks in the on-track condition, but also under various conditions the tracking system sometimes cannot operate in locked servo control.
As mentioned above, tracking systems in prior art have serious drawbacks when starting their tracking for data recording or retrieving in the optical disk memory apparatus.